Quantum tunnelling

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v t e

In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a

, should not be passable due to the object not having sufficient energy to pass or surmount the barrier.

Tunneling is a consequence of the wave nature of matter, where the quantum wave function describes the state of a particle or other physical system, and wave equations such as the Schrödinger equation describe their behavior. The probability of transmission of a wave packet through a barrier decreases exponentially with the barrier height, the barrier width, and the tunneling particle's mass, so tunneling is seen most prominently in low-mass particles such as electrons or protons tunneling through microscopically narrow barriers. Tunneling is readily detectable with barriers of thickness about 1–3 nm or smaller for electrons, and about 0.1 nm or smaller for heavier particles such as protons or hydrogen atoms.^[1] Some sources describe the mere penetration of a wave function into the barrier, without transmission on the other side, as a tunneling effect, such as in tunneling into the walls of a finite potential well.^[2][3]

Tunneling plays an essential role in physical phenomena such as

The effect was predicted in the early 20th century. Its acceptance as a general physical phenomenon came mid-century.[7]

Introduction to the concept

Quantum tunnelling falls under the domain of

can be compared to a ball trying to roll over a hill. Quantum mechanics and classical mechanics differ in their treatment of this scenario.

Classical mechanics predicts that particles that do not have enough energy to classically surmount a barrier cannot reach the other side. Thus, a ball without sufficient energy to surmount the hill would roll back down. In quantum mechanics, a particle can, with a small probability, tunnel to the other side, thus crossing the barrier. The reason for this difference comes from treating matter as having properties of waves and particles.

The tunnelling problem

The wave function of a physical system of particles specifies everything that can be known about the system.^[8] Therefore, problems in quantum mechanics analyze the system's wave function. Using mathematical formulations, such as the Schrödinger equation, the time evolution of a known wave function can be deduced. The square of the absolute value of this wave function is directly related to the probability distribution of the particle positions, which describes the probability that the particles would be measured at those positions.

As shown in the animation, a wave packet impinges on the barrier, most of it is reflected and some is transmitted through the barrier. The wave packet becomes more de-localized: it is now on both sides of the barrier and lower in maximum amplitude, but equal in integrated square-magnitude, meaning that the probability the particle is somewhere remains unity. The wider the barrier and the higher the barrier energy, the lower the probability of tunneling.

Some models of a tunneling barrier, such as the rectangular barriers shown, can be analysed and solved algebraically.^[9]: 96 Most problems do not have an algebraic solution, so numerical solutions are used. "Semiclassical methods" offer approximate solutions that are easier to compute, such as the WKB approximation.

History

The Schrödinger equation was published in 1926. The first person to apply the Schrödinger equation to a problem which involved tunneling between two classically allowed regions through a potential barrier was

In 1927,

A great success of the tunnelling theory was the mathematical explanation for alpha decay, which was developed in 1928 by George Gamow and independently by Ronald Gurney and Edward Condon.^{[12][13][14][15]} The latter researchers simultaneously solved the Schrödinger equation for a model nuclear potential and derived a relationship between the half-life of the particle and the energy of emission that depended directly on the mathematical probability of tunneling. All three researchers were familiar with the works on field emission,^[10] and Gamow was aware of Mandelstam and Leontovich's findings.^[16]

In the early days of quantum theory, the term tunnel effect was not used, and the effect was instead referred to as penetration of, or leaking through, a barrier. The German term wellenmechanische Tunneleffect was used in 1931 by Walter Schottky. The English term tunnel effect entered the language in 1932 when it was used by Yakov Frenkel in his textbook.^[10]

In 1957

In 1981, Gerd Binnig and Heinrich Rohrer developed a new type of microscope, called scanning tunneling microscope, which is based on tunnelling and is used for imaging surfaces at the atomic level. Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986 for their discovery.^[19]

Applications

Tunnelling is the cause of some important macroscopic physical phenomena.

Solid-state physics

Electronics

Tunnelling is a source of current leakage in

.

Cold emission

Cold emission of

These materials are important for flash memory, vacuum tubes, and some electron microscopes.

Tunnel junction

A simple barrier can be created by separating two conductors with a very thin

.

Tunnel diode

resonant tunnelling diode

device, based on the phenomenon of quantum tunnelling through the potential barriers

Because the tunnelling current drops off rapidly, tunnel diodes can be created that have a range of voltages for which current decreases as voltage increases. This peculiar property is used in some applications, such as high speed devices where the characteristic tunnelling probability changes as rapidly as the bias voltage.[23]

The

Tunnel field-effect transistors

A European research project demonstrated

Conductivity of crystalline solids

While the

Scanning tunneling microscope

Main article:

Scanning tunnelling microscope

The scanning tunnelling microscope (STM), invented by Gerd Binnig and Heinrich Rohrer, may allow imaging of individual atoms on the surface of a material.^[21] It operates by taking advantage of the relationship between quantum tunnelling with distance. When the tip of the STM's needle is brought close to a conduction surface that has a voltage bias, measuring the current of electrons that are tunnelling between the needle and the surface reveals the distance between the needle and the surface. By using piezoelectric rods that change in size when voltage is applied, the height of the tip can be adjusted to keep the tunnelling current constant. The time-varying voltages that are applied to these rods can be recorded and used to image the surface of the conductor.^[21] STMs are accurate to 0.001 nm, or about 1% of atomic diameter.^[24]

Nuclear physics

Nuclear fusion

Quantum tunnelling is an essential phenomenon for nuclear fusion. The temperature in

Radioactive decay

Radioactive decay is the process of emission of particles and energy from the unstable nucleus of an atom to form a stable product. This is done via the tunnelling of a particle out of the nucleus (an electron tunneling into the nucleus is

Quantum tunnelling may be one of the mechanisms of hypothetical proton decay.^[28][29]

Chemistry

Kinetic isotope effect

In chemical kinetics, the substitution of a light isotope of an element with a heavier one typically results in a slower reaction rate. This is generally attributed to differences in the zero-point vibrational energies for chemical bonds containing the lighter and heavier isotopes and is generally modeled using transition state theory. However, in certain cases, large isotopic effects are observed that cannot be accounted for by a semi-classical treatment, and quantum tunnelling is required. R. P. Bell developed a modified treatment of Arrhenius kinetics that is commonly used to model this phenomenon.^[30]

Astrochemistry in interstellar clouds

By including quantum tunnelling, the astrochemical syntheses of various molecules in interstellar clouds can be explained, such as the synthesis of molecular hydrogen, water (ice) and the prebiotic important formaldehyde.^[27] Tunnelling of molecular hydrogen has been observed in the lab.^[31]

Quantum biology

Quantum tunnelling is among the central non-trivial quantum effects in

Spontaneous mutation occurs when normal DNA replication takes place after a particularly significant proton has tunnelled.

Mathematical discussion

The Schrödinger equation

The time-independent Schrödinger equation for one particle in one dimension can be written as

or

where

• ${\displaystyle \hbar }$ is the reduced
  Planck's constant
  ,
• m is the particle mass,
• x represents distance measured in the direction of motion of the particle,
• Ψ is the Schrödinger wave function,
• V is the potential energy of the particle (measured relative to any convenient reference level),
• E is the energy of the particle that is associated with motion in the x-axis (measured relative to V),
• M(x) is a quantity defined by V(x) − E which has no accepted name in physics.

The solutions of the Schrödinger equation take different forms for different values of x, depending on whether M(x) is positive or negative. When M(x) is constant and negative, then the Schrödinger equation can be written in the form

The solutions of this equation represent travelling waves, with phase-constant +k or -k. Alternatively, if M(x) is constant and positive, then the Schrödinger equation can be written in the form

The solutions of this equation are rising and falling exponentials in the form of

. When M(x) varies with position, the same difference in behaviour occurs, depending on whether M(x) is negative or positive. It follows that the sign of M(x) determines the nature of the medium, with negative M(x) corresponding to medium A and positive M(x) corresponding to medium B. It thus follows that evanescent wave coupling can occur if a region of positive M(x) is sandwiched between two regions of negative M(x), hence creating a potential barrier.

The mathematics of dealing with the situation where M(x) varies with x is difficult, except in special cases that usually do not correspond to physical reality. A full mathematical treatment appears in the 1965 monograph by Fröman and Fröman. Their ideas have not been incorporated into physics textbooks, but their corrections have little quantitative effect.

The WKB approximation

The wave function is expressed as the exponential of a function:

where

${\displaystyle \Phi '(x)}$ is then separated into real and imaginary parts:

where A(x) and B(x) are real-valued functions.

Substituting the second equation into the first and using the fact that the real part needs to be 0 results in:

To solve this equation using the semiclassical approximation, each function must be expanded as a power series in ${\displaystyle \hbar }$. From the equations, the power series must start with at least an order of ${\displaystyle \hbar ^{-1}}$ to satisfy the real part of the equation; for a good classical limit starting with the highest power of

possible is preferable, which leads to and with the following constraints on the lowest order terms, and

At this point two extreme cases can be considered.

Case 1

If the amplitude varies slowly as compared to the phase ${\displaystyle A_{0}(x)=0}$ and

which corresponds to classical motion. Resolving the next order of expansion yields

Case 2

If the phase varies slowly as compared to the amplitude, ${\displaystyle B_{0}(x)=0}$ and

which corresponds to tunneling. Resolving the next order of the expansion yields

In both cases it is apparent from the denominator that both these approximate solutions are bad near the classical turning points ${\displaystyle E=V(x)}$. Away from the potential hill, the particle acts similar to a free and oscillating wave; beneath the potential hill, the particle undergoes exponential changes in amplitude. By considering the behaviour at these limits and classical turning points a global solution can be made.

To start, a classical turning point, ${\displaystyle x_{1}}$ is chosen and ${\displaystyle {\frac {2m}{\hbar ^{2}}}\left(V(x)-E\right)}$ is expanded in a power series about ${\displaystyle x_{1}}$:

Keeping only the first order term ensures linearity:

Using this approximation, the equation near ${\displaystyle x_{1}}$ becomes a differential equation:

This can be solved using Airy functions as solutions.

Taking these solutions for all classical turning points, a global solution can be formed that links the limiting solutions. Given the two coefficients on one side of a classical turning point, the two coefficients on the other side of a classical turning point can be determined by using this local solution to connect them.

Hence, the Airy function solutions will asymptote into sine, cosine and exponential functions in the proper limits. The relationships between ${\displaystyle C,\theta }$ and ${\displaystyle C_{+},C_{-}}$ are

and

${\displaystyle C_{-}=-C\sin {\left(\theta -{\frac {\pi }{4}}\right)}}$

With the coefficients found, the global solution can be found. Therefore, the

for a particle tunneling through a single potential barrier is

where ${\displaystyle x_{1},x_{2}}$ are the two classical turning points for the potential barrier.

For a rectangular barrier, this expression simplifies to:

Faster than light

Some physicists have claimed that it is possible for spin-zero particles to travel faster than the

Other physicists, such as Herbert Winful,^[38] disputed these claims. Winful argued that the wave packet of a tunnelling particle propagates locally, so a particle can't tunnel through the barrier non-locally. Winful also argued that the experiments that are purported to show non-local propagation have been misinterpreted. In particular, the group velocity of a wave packet does not measure its speed, but is related to the amount of time the wave packet is stored in the barrier. But the problem remains that the wave function still rises inside the barrier at all points at the same time. In other words, in any region that is inaccessible to measurement, non-local propagation is still mathematically certain.

A 2020 experiment, overseen by Aephraim M. Steinberg, showed that particles should be able to tunnel at apparent speeds faster than light.^[39][40]

Dynamical tunneling

The concept of quantum tunneling can be extended to situations where there exists a quantum transport between regions that are classically not connected even if there is no associated potential barrier. This phenomenon is known as dynamical tunnelling.^[41][42]

Tunnelling in phase space

The concept of dynamical tunnelling is particularly suited to address the problem of quantum tunnelling in high dimensions (d>1). In the case of an integrable system, where bounded classical trajectories are confined onto tori in phase space, tunnelling can be understood as the quantum transport between semi-classical states built on two distinct but symmetric tori.^[43]

Chaos-assisted tunnelling

In real life, most systems are not integrable and display various degrees of chaos. Classical dynamics is then said to be mixed and the system phase space is typically composed of islands of regular orbits surrounded by a large sea of chaotic orbits. The existence of the chaotic sea, where transport is classically allowed, between the two symmetric tori then assists the quantum tunnelling between them. This phenomenon is referred as chaos-assisted tunnelling.^[44] and is characterized by sharp resonances of the tunnelling rate when varying any system parameter.

Resonance-assisted tunnelling

When ${\displaystyle \hbar }$ is small in front of the size of the regular islands, the fine structure of the classical phase space plays a key role in tunnelling. In particular the two symmetric tori are coupled "via a succession of classically forbidden transitions across nonlinear resonances" surrounding the two islands.^[45]

Related phenomena

Several phenomena have the same behavior as quantum tunnelling. Two examples are

]

These effects are modeled similarly to the

solutions in medium B.

In

and medium B is the potential barrier. These have an incoming wave and resultant waves in both directions. There can be more mediums and barriers, and the barriers need not be discrete. Approximations are useful in this case.

A classical wave-particle association was originally analyzed as analogous to quantum tunneling,[47] but subsequent analysis found a fluid dynamics cause related to the vertical momentum imparted to particles near the barrier.^[48]

See also

• Dielectric barrier discharge
• Field electron emission
• Holstein–Herring method
• Proton tunneling
• Quantum cloning
• Superconducting tunnel junction
• Tunnel diode
• Tunnel junction
• White hole

References

Further reading

• Binney, James; Skinner, D. (2010). The Physics of Quantum Mechanics: An Introduction (3rd ed.). Cappella Archive.
  ISBN 978-1-902918-51-8
  .
• Fröman, N.; Fröman, P.-O. (1965). JWKB Approximation: Contributions to the Theory. Amsterdam: North-Holland.
• Griffiths, David J. (2004). Introduction to Quantum Mechanics (2nd ed.). Prentice Hall.
  ISBN 978-0-13-805326-0
  .
• Liboff, Richard L. (2002). Introductory Quantum Mechanics. Addison-Wesley.
  ISBN 978-0-8053-8714-8
  .
• Müller-Kirsten, H. J. W. (2012). Introduction to Quantum Mechanics: Schrödinger Equation and Path Integral, 2nd ed. Singapore: World Scientific.
• Razavy, Mohsen (2003). Quantum Theory of Tunneling. World Scientific.
  ISBN 978-981-238-019-7
  .
• Vilenkin, Alexander; Vilenkin, Alexander; Winitzki, Serge (2003). "Particle creation in a tunneling universe".
  S2CID 118969589
  .
• Wolf, E. L. (2012). Principles of Electron Tunneling Spectroscopy. International series of monographs on physics (2 ed.). Oxford: Oxford University Press.
  ISBN 978-0-19-958949-4
  .

External links

Wikimedia Commons has media related to Quantum tunneling.

• Animation, applications and research linked to tunnel effect and other quantum phenomena (Université Paris Sud)
• Animated illustration of quantum tunneling
• Animated illustration of quantum tunneling in a RTD device
• Interactive Solution of Schrodinger Tunnel Equation